The precise measurement of optical waveguides, such as optical fiber, integrated devices, and waveguides within opto-electronic devices, is an area of critical interest in telecommunications, transmission, opto-electronic device testing, aeronautics, and many other fields utilizing optical waveguides. Non-real time methods of Optical Time Domain Reflectometry (OTDR) widely used to determine measurement, tend to be slow and often produce results which are less than desirable. Standard scanning wavelength Fourier transform OTDR methods are time consuming and unable to extract real time or near real time reflectivity data from working components such as fast modulators. As well, these standard methods require a computer to perform the Fourier transform to obtain the spatial resolution. Similarly, interferometric methods require holographic stability during the measurement and most demonstrations have been restricted to laboratory settings.
In Optics Letters, Vol. 4 No. 2 February 1979, pp. 58-59, R. Normandin (the present inventor) et al, reported the non-linear mixing of oppositely propagating guided waves. The resultant field was coupled to radiation modes and propagated in a direction perpendicular to the waveguide surface, in the case of equal frequency fundamentals. In subsequent articles, its application to picosecond signal processing, the creation of all optical transient digitizers and spectrometers demonstrated the potential usefulness of this work. (See Appl. Phys. Lett. 36 (4), Feb. 15, 1980, pp. 253-255 by R. Normandin et al; 40 (9), 1982, pp. 759-761 by R. Normandin et al, and "Integrated Optical Circuits and Components" edited by L. D. Hutcheson, Dekker Inc., New York, U.S.A., Chapter 9, by G. I. Stegeman et at.). The overlap of two oppositely propagating fields will give rise to a non-linear polarization source at the sum frequency. In bulk media such a process is nonradiative due to the simultaneous requirement of energy and momentum conservation in all directions. This is not the case in a waveguide geometry.
Unfortunately, since the waves do not grow with distance, (no phase matching) the resultant fields are much weaker than that obtained in a traditional second harmonic generation device. Therefore, this non-linear interaction has remained largely a laboratory curiosity. However in U.S. Pat. No. 5,051,617, entitled Multilayer Semiconductor Waveguide Device for Sum Frequency Generation From Contra-Propagating Beams, the present inventor has increased this interaction by factors of 10.sup.7 to obtain efficient conversion in the visible region. Thus, with the invention disclosed in U.S. Pat. No. 5,051,617, ultra fast subpicosecond samplers and monolithic high resolution spectrometers are possible in the context of fiber optic communication systems and optoelectronic integrated circuitry.
When two guided fundamental wavelengths are identical, oppositely propagating and traveling in the same collinear and one dimensional path, a radiated harmonic signal is observed in a direction perpendicular to the surface of the waveguide.
It is an object of the invention to utilize this property of a non-linear waveguide to provide means of measuring short pulses without the need for fast electronic circuitry.
It is a further object of the invention to utilize this property of a non-linear waveguide to determine a variance of length of a coupled waveguide to within small increment.
It is a further object of the invention, to provide a reliable, accurate system for monitoring a change in length of an optical waveguide.
It is yet another object of the invention to provide a non-linear OTDR system for use with a monolithically integrated device without any moving parts, to remotely probe the time varying features of opto-electronic integrated circuits in a working environment.
In accordance with the invention, there is provided, a method of detecting the location or profile and time duration of a collision of two light pulses within a nonlinear waveguide, comprising the steps of: providing first and second oppositely propagating pulses into the non-linear waveguide, and detecting the location along a surface of the waveguide of sum frequency light radiated from the surface, that detected location being indicative of the location of the collision of the two light pulses within the nonlinear waveguide or being indicative of the spatial envelope related to the time profile of the tow light pulses.
In accordance with the invention, there is further provided a system for detecting the location of a collision of two light pulses comprising: a nonlinear waveguide; means for providing a first and a second oppositely propagating pulse into the nonlinear waveguide; and, means detecting the location along the waveguide surface of sum frequency light radiated from the surface, the location of the radiated sum frequency light being indicative of the location of the collision within the waveguide.
In accordance with another aspect of the invention, there is provided, an optical device for obtaining the temporal convolution of a first and second input pulse comprising: a non-linear waveguide; a first waveguide for being coupled to an end of the nonlinear waveguide; a second waveguide for being coupled to an other end of the nonlinear waveguide; and, variable delay means positioned in series with the second waveguide for delaying an optical signal; means for launching into the first and second waveguides, first and second light pulses; means for coupling a device to be tested in series with the first waveguide, and, detector means for detecting at the position at the surface of the nonlinear waveguide of sum frequency light radiated from the nonlinear waveguide, the detected position relating to the location of a collision of the first and second light pulses within the nonlinear waveguide.